When you say that "all of the transformers" hummm . . . are you really sure? It only takes one for it to really, really sound that way sometimes.
The most common cause of hum and buzz in power transformers is loose core laminations in the transformer itself, or vibrating/loose metalwork around it -- and there's usually one particular component that's the major offender. If you do narrow it down to a certain piece, you might contact the manufacturer . . . they may be aware of a noisy production batch, or have a modification to the mounting to specifically deal with the problem.
The most common cause of hum and buzz in power transformers is loose core laminations in the transformer itself, or vibrating/loose metalwork around it -- and there's usually one particular component that's the major offender. If you do narrow it down to a certain piece, you might contact the manufacturer . . . they may be aware of a noisy production batch, or have a modification to the mounting to specifically deal with the problem.